Chemical reactions Study Guide

Chemical reactions involve the breaking of existing bonds in reactants and the formation of new bonds in products. Energy is absorbed from the surroundings to break bonds, making bond breaking an endothermic process. Conversely, energy is released to the surroundings when new bonds are formed, making bond formation an exothermic process. The overall energy change of a reaction, often called the enthalpy change (ΔH), is determined by the difference between the total energy absorbed during bond breaking and the total energy released during bond formation. If more energy is absorbed to break bonds than is released when new bonds form, the reaction is endothermic, and ΔH will be positive, indicating a net absorption of energy from the surroundings. This often leads to a decrease in temperature of the surroundings. If, however, more energy is released during bond formation than is absorbed to break bonds, the reaction is exothermic, and ΔH will be negative, indicating a net release of energy to the surroundings. This typically results in an increase in the temperature of the surroundings. To calculate the overall energy change, you sum the bond energies for all bonds broken in the reactants and sum the bond energies for all bonds formed in the products. The general equation is: Energy Change (ΔH) = (Sum of energy absorbed for bond breaking in reactants) - (Sum of energy released for bond formation in products). Bond energies are average values and can vary slightly depending on the specific molecule. These calculations provide an estimation of the energy change.

Stoichiometry is the quantitative relationship between reactants and products in a balanced chemical equation. It is based on the law of conservation of mass, which states that matter cannot be created or destroyed in a chemical reaction. This means the total mass of reactants must equal the total mass of products. The coefficients in a balanced chemical equation represent the molar ratios of the substances involved. For example, in the reaction 2H₂ + O₂ → 2H₂O, two moles of hydrogen react with one mole of oxygen to produce two moles of water. These molar ratios allow us to calculate the mass of product formed from a given mass of reactant, or the mass of reactant needed to produce a certain mass of product.

To perform stoichiometric calculations using masses, you first convert the given masses into moles using the formula: moles = mass / molar mass (Ar or Mr). Then, use the molar ratios from the balanced equation to find the moles of the desired substance. Finally, convert these moles back into mass. It is crucial to have a balanced chemical equation to ensure accurate molar ratios.

A limiting reactant (or limiting reagent) is the reactant that is completely consumed in a chemical reaction and therefore determines the maximum amount of product that can be formed. Once the limiting reactant is used up, the reaction stops, even if other reactants are still present in excess. The reactant that is not completely used up is called the excess reactant. Identifying the limiting reactant is essential for predicting the theoretical yield of a reaction. To find the limiting reactant, calculate the moles of each reactant and then compare their molar ratios to the ratios in the balanced equation. The reactant that would run out first, based on these ratios, is the limiting reactant. For example, if you have 10 moles of hydrogen and 2 moles of oxygen for the reaction 2H₂ + O₂ → 2H₂O, you need 2 moles of H₂ for every 1 mole of O₂. For 2 moles of O₂, you would need 4 moles of H₂. Since you have 10 moles of H₂, hydrogen is in excess and oxygen is the limiting reactant.